Aluminum ageing in operating

[LauriRauhala]

Hello,

I am designing a centrifugal heat pump, which compresses air. I'm considering making the impeller from aluminum, probably from 7075, since the stresses rise to about 385 MPa when rotating at high speeds. The input temperature of the air is 85 degrees Celcius and output 125 degrees Celcius. I am wondering how the impeller will age in the process over the years, and since the stress is quite high, is there a risk that the ageing will decrease yield strength lower than my max stress, 385 MPa?

Any tips or insights are very welcome, thanks in advance!

Best regards,
Lauri

 

[SWComposites]

7075-T6 is very susceptible to stress corrosion cracking.

 

[EdStainless]

How is the impeller being manufactured? Cast? Injection molded? Machined?
I believe that the last ones that I saw were thixocast (slush) in a variation of a 6100 alloy.
The alloy composition will depend on this also.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[LauriRauhala]

The impeller is machined from a block

 

[EdStainless]

As I re-read this does it say that the design stress equals the yield strength?
If so then this will just last a very short time.
Your operating stress should be about 1/2 of the yield strength at the outlet temperature.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

Industrial diesel engine turbochargers are largely adopting titanium for compressors due to high temperatures. Creep determines lifetime for aluminum compressors at high temperature.

 

[LauriRauhala]

Thanks for the answers!

EdStainless, not quite the FEA tells about 385 MPa max stress, while the yield strength is about 550 MPa. My worry is that the 125 degrees Celsius ages the aluminum fast to decrease yield strength, but there is not much information on that.

TugBoatEng, yes that's what I heard and also why I reached out to the forums. Do you know any studies or references on that? I tried to find, but they are mostly focused on either fast artificial ageing or natural aging in room temperatures. Not aging in operation with relatively high temperatures.

 

[EdStainless]

from MMPDS-04
3.7.7.0 Comments and Properties — 7075 is a high-strength Al-Zn-Mg-Cu alloy and is available in a wide variety of product forms. It is also available in several types of tempers, T6, T73, and T76
type. The T6 temper has the highest strength but lowest toughness and resistance to stress corrosion cracking.
Since toughness decreases with a decrease in temperature, the T6 temper is not generally recommended for
cryogenic applications. As shown in Table 3.1.2.3.1(a), 7075-T6 rolled plate, rod and bar, extruded shapes,
and forgings have a “D” SCC rating. This is the lowest rating and means that SCC failures have occurred
in service or would be anticipated if there is any sustained stress. In-service failures are caused by stress
produced by any combination of sources, including solution heat treatment, straightening, forming, fit-up,
clamping, sustained service loads, or high service compression stresses that produce residual tensile stresses.
These stresses may be tension or compression, as well as the stresses due to the Poisson effect, because the
actual failures are caused by the resulting sustained shear stresses. Pin-hole flaws in corrosion protection are
sufficient for SCC. The T73 temper provides for much improved stress-corrosion resistance over T6 temper
with a decrease in strength. The T76 temper provides for improved exfoliation resistance and limited stress
corrosion resistance over T6 temper with some decrease in strength.

I can't copy all of the charts but you need to look.
At 125C your strength (without any aging factors) will be about 76% of the RT strength.
They do have time factor curves, but at 125C the effect isn't much.
There are also fatigue curves, and the strengths are low such as no more than half of the original strength.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[WKTaylor]

Generally speaking the service temperature limit for 7xxx-Tanything is generally 250F [122C].

The only alloys I could think of that would run all-day at 250F and maintain suitable strength, toughness and SCC/EXCO resistance is... in order of my preference...

2219-T851 Plate or -T852 forged-block...
It has been used in critical cryogenic applications as well as those applications in which high strength and creep resistance at relatively high temperatures (400 F to 600 F) are required.

CAUTION1. At ~250F, Per MMPDS-17... 2219-T8xx retains ~86% of RT FTU/FTY ... so redesign for lower strength [at temperature] is imperative.

Also, 2024-T8xx plate or 2124-T8xx plate might work well at 250F with similar reduced FTU/FTY... but would required design changes for reduced steady state stresses 'same as 2219-T8xxx'. These alloys tend to have an upper service temp limit of ~350F.

On-the-other-hand Titanium 6Al-4V Annealed will barely break a sweat and have plenty of MS available... at the 55-KSI and 250F.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[TugboatEng]

Diesel engine customers are the cheapest you'll ever meet. With modern emissions rules were replacing machinery every 15-20k hours. At that rate we can still afford titanium compressors so you can too.

 

[LauriRauhala]

Thank you all very much for the answers. Titanium is a choice I'm seriously considering. This all gives me a lot to digest.

 

[WKTaylor]

LR... ONLY CAUTION... using any suitable material... Duhhh....

Ensure Your design accounts for compatible [rotating] compressor and it's housing materials. Ensure You design for the entire thermal range you will encounter in-service. Expansion and contraction will be aggravate by ambient [inlet] air temperature... and obviously... compressor loads and rotational forces. The combinations must be considered.

For EXAMPLE...

Compressor OFF at start-up... lowest anticipated 'cold-start/run' temp. For aircraft this might be -40F[-40C] down to -65F[-53C]... or worst-case -100F[-74C].

Compressor OFF... and highest operating temp running'... based on the highest in-take air temperature. For aircraft this compressor may have to operate [start/run] on the ramp in [like Saudi Arabia] at +140F[60C]... or worst-case +160F[71C].

Reason... when thermal miss-matches occur, then 'leakage' due to excessive gapping... or 'rubbing' due over-lapping contact is possible. Gaps result in reduced performance... rubbing can be catastrophic.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[EdStainless]

The other caution is that Ti (all alloys, and Al for that matter) have much lower fatigue strength than people familiar to steels would expect. Generous root radius, care with surface finish, and of course the design load are going to be important.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed